Leukocytes, or white blood cells, are cells of the immune system that protect the body against infectious disease and toxins. There are several different and functionally diverse kinds of leukocytes in humans, all however are derived from a common pluripotent bone marrow-derived stem cell. In a healthy person there are about 8×106 leukocytes per milliliter of blood. These leukocytes are comprised of basophils, lymphocytes, neutrophils, eosinophils, macrophages and monocytes. The monocyte fraction accounts only for about 2-8% of all leukocytes.
The macrophage was first observed by physicians during the mid-nineteenth century, when these cells were described in battlefield wounds and in association with areas of chronic inflammation. Extensive study of these cells since has revealed that the monocyte-macrophage system plays several crucial functions: in tissue maintenance and repair; immune regulation; and in control and elimination of pathogens.
Monocytes are only temporary residents in the blood. After development in the bone marrow, monocytes circulate in the bloodstream where they have a half-life of a few days. During an acute inflammatory response, the half-life may be only a few hours. Egress of monocytes from the blood into tissues is associated with an increase in cell size together with a larger, lighter-staining nucleus, indicating intense RNA expression. These tissue-infiltrated cells are termed macrophages (or histiocytes). These cells are phagocytic and mobile.
When monocytes take up residence in various tissues they may persist there for years. In the spleen, macrophages are involved in recycling old red blood cells; in the skin (Langerhans cells), they are involved in keratinization of the epidermis; in the liver (Kupfer cells), they manage degradation of toxins; in the intima of the artery (foam cells), they are involved in the formation of atherosclerotic plaques; in the joint, they differentiate into cells (type A synoviocytes) that are involved in maintenance of the joint; in lymph nodes, they function as antigen presenting cells (dendritic cells), stimulating adaptive immune responses; in bone, they regulate resorption of bone mass (osteoclasts); while in the central nervous system (glial cells), they act as sentinel cells and are involved in neuroendocrine homeostasis. Monocytes and the cells that they differentiate were once commonly referred to as the reticuloendothelial system. While no longer popularly referred to as such, this nomenclature addressed the multiple roles of monocytes in the homeostasis of tissues and organ systems.
In order for monocytes to perform their myriad functions in tissues, they first must exit the blood vessel and enter tissue. To accomplish this, monocytes, once activated, adhere to the endothelial cells of the blood vessel wall and extravasate, or penetrate, the cell matrix that forms the vessel wall. This extravasation process is not unique to monocytes. The molecular mechanism of adherence involves the adhesion molecules CD11a, CD11b, CD11c/CD18, common to virtually all monocytes, as well as lymphocytes and neutrophils. Since extravasation of leukocytes into tissues is the first step in many disease processes, attempts have been made to block this process with antibodies. Extreme care, however, must be taken not to effect a generalized blockade of this process. The human genetic disease known as Leukocyte adhesion deficiency (LAD), a defect of the CD11/CD18 system, results in severe immunosuppression. Individuals with LAD die from opportunistic infections if left unprotected from pathogens. Consequently only a very selective blockade of this process is acceptable. One such treatment involves the use of an antibody Efalizumab, which targets CD11c, which is expressed as CD11c/CD18 on certain leukocyte subsets, such a T lymphocytes. However, no means for selectively blocking monocyte function has been devised.
As distinguished from other leukocytes, monocytes represent a unique cellular compartment. Yet monocytes themselves are a functionally heterogeneous population of cells. In humans, these populations may be generally divided into two groups based on expression of cell surface markers: (1) the major population defined as CD14 high expressing (CD14++) and (2) the minor population defined as CD14 and CD16 co-expressing (CD14+CD16+). The latter is known as a proinflammatory subset of monocytes and is associated with numerous inflammation-associated diseases including atherosclerosis, cancer, rheumatoid arthritis, and Alzheimer's disease.
Therapeutic strategies involving leukocytopheresis to remove pro-inflammatory CD14+CD16+ monocytes from patients have been attempted. For example, extracorporeal elimination of CD14+CD16+ monocytes was successful in treating ulcerative colitis (Kanai et al. Inflamm. Bowel Dis. 2007 March; 13(3):284-90). This approach, however, resulted in significant morbidity and risk to the patient and is not suitable for treatment of chronic illness. Moreover, leukocytophoresis is not highly selective for CD14+CD16+ monocytes but rather depletes most monocyte populations. Therefore, leukocytophoresis is not suitable for treatment of diseases where sustained, long-term depletion is necessary, since this is expected to cause dangerous immunosuppression. Rather a highly selective means of targeting CD14+CD16+ is necessary.
A preferred approach would be to selectively target the CD14+CD16+ proinflammatory monocyte subset with an agent that specifically downregulates that subset's function. Heretofore, no practical means to precisely target these cells has been identified. Although these cells are characterized by expression of CD14 and CD16 surface proteins, these are not appropriate for targeting therapy. CD14 would not be an appropriate target because it is expressed by all monocytes as well as other cells populations, such as neutrophils. Modulating function of the entire population of CD14-expressing cells would result in unacceptable risk of severe immunosuppression. Targeting CD16 (FcgRIII) would also not be appropriate because, in addition to expression on monocytes, it is expressed on a number of other crucial immune cells including B and T lymphocytes. Accordingly, a good marker for targeting those CD14+CD16+ monocytes has been lacking.